Known tissue growth stimulators generally fall into at least two broad categories. The first category consists of implantable direct current ("DC") devices. The generator of such stimulators are implanted in the body near the site of tissue damage, a bone fracture or a fusion. A cathode typically exits the case of the DC stimulator leading directly to the injury site. The stimulator case acts as the anode. Electronics within the stimulator cause a direct current to flow between cathode and anode and thereby through the tissue site bone fracture or fusion site generally. A second class of tissue growth stimulators are external or noninvasive stimulators. These stimulators are aligned adjacent to a tissue site, bone fracture or fusion site outside the body. Typically, these devices generate either a pulsed electromagnetic field ("PEMF") or a 60 kHz sinusoidal electric field to promote healing at the injury site. These subgroups of noninvasive stimulators are referred to as PEMF and capacitive coupling stimulators.
There are numerous disadvantages associated with known direct current implantable tissue growth stimulators. In general, the DC characteristics of these stimulators require the leads to be routed directly to the injury site. It is believed that the chemical change at the cathode surface induces tissue growth. Additionally, the cathode (or cathodes) is usually embedded in the tissue, fracture or bone graft mass. It may be required during explant of the stimulator that the implanted cathode be left in the body. This may be necessary after the tissue heals and encapsulates the cathode originally implanted into the tissue mass. If the cathode is damaged, or otherwise becomes inoperative, extensive surgery will be required to replace it at the tissue site. This increases the likelihood of surgical complications such as infection. Also, the DC stimulator and its cathode will degrade imaging results due to their proximity to relevant body structures. Imaging techniques such as magnetic resonance imaging, computer-aided tomography and x-ray photography will all be affected.
External tissue growth stimulators also have disadvantages associated with them. Because of their placement outside the human body, these stimulators are vulnerable from ambulatory or semiambulatory patients. Their movement, whether intentional or inadvertent, may cause damage to the unit. These devices are also cumbersome and usually require the patient to operate them. This creates a question of patient compliance and ultimately of stimulator effectiveness. Furthermore, capacitive coupled stimulators require a conductive gel between the patient's skin and each electrode. This gel must be replaced often and is known to cause skin irritation.
Most known stimulators simply are turned on by the manufacturer and turned off when the stimulator battery dies or the power supply is otherwise disconnected. U.S. Pat. No. 4,414,979 to Hirshorn, entitled "Monitorable Bone Growth Stimulator" issued Nov. 15, 1983, discloses an implantable DC bone growth stimulator which transmits pulses of electromagnetic energy at a rate proportional to the current being delivered to the injury site. This allows some degree of monitorability of the energy delivered to the bone site. However, other parameters of tissue growth stimulators are also of interest. It may be important for the attending physician to know the mode of operation of the stimulator, the expected lifetime of the associated stimulator battery, and the condition of the leads. Conversely, it is also desirable to be able to program certain operating modes of a tissue growth stimulator. Such capability is particularly important with implantable tissue growth stimulators since they are inaccessible otherwise. Such monitorability and programmability have not been available with prior implantable stimulators.
Therefore, a need has arisen for a tissue growth stimulator which is implantable, which is easily replaced and completely removable after use, which is both monitorable and programmable during operation, which does not require patient participation, and which does not interfere with imaging results.